1. Field of the Invention
The present invention relates to an apparatus for reading letters, characters and/or graphic patterns and converting them to electric signals, and more particularly to a photoelectric converter apparatus in a pattern reading system which converts letters, characters and/or graphic patterns to electric signals by photoelectrically converting light reflected from a document on which letters, characters and/or graphic patterns are marked, and injected on the picture cells.
2. Description of the Prior Art
In an apparatus for converting characters and graphic patterns to electric signals such as facsimile or optical character reader (OCR), a charge storage type photoelectric converter device or a charge coupled device (CCD) has been used as a photoelectric converter apparatus of a pattern reading system for converting the character and graphic patterns to the electric signals. The charge storage type photoelectric converter device may typically comprise a photo-diode array in which charge is stored by radiation of light.
The pattern reading system comprises a light source for illuminating a document having characters and/or graphic patterns marked thereon, a photoelectric converter apparatus having a number of linearly arranged photo-diodes and a readout circuit for serially reading out charges stored in the photo-diodes.
The light emitted from the light source and reflected by the document is condensed by a lens and directed to and imaged on the photo-diode array of the photoelectric converter apparatus. After a predetermined period for the storage of the charge, the charge is read by the readout circuit. The above operation is repeated a number of times while sequentially displacing the document.
A white area on the document reflects a large quantity of light and hence a photo-diode on which is imaged a white area of the image stores a large amount of charge. On the other hand, a photo-diode on which is imaged a black area of the image stores a small amount of charge. In this manner, an electrical signal representative of a density of the document is produced.
The amounts of charges may be handled in the form of an analog quantity but in many cases they are converted to "1" or "0" (white/black) digital signals by comparing the detected signal with a predetermined threshold level.
In a conventional apparatus, when the charge storage type CCD photoelectric converter device (hereinafter referred to as the CCD photo-sensor) is driven, unnecessary charges stored in the respective picture cells of the diode array of the photo-sensor are transferred to a CCD analog shift register once for each start of a scan command to sweep away the unnecessary stored charges from the diode array and then text readout data is stored in the diode array. However, when a large quantity of light has been irradiated or an interval between adjacent starts of a scan command is long, the charges in the photo-diode array may not be completely swept away in a predetermined short period and the signal charges may be superposed on or added to the unnecessary charges left behind in the respective picture cells. Further, since the capacity of each cell of the shift register is smaller than that of the image-sensing photodiode array, the cells of the shift register may be saturated if the amount of charge transferred from the image sensor is large. In such cases, the readout data does not faithfully reflect the data on the text.
In a CCD photosensor drive system, a transfer gate may be opened to sweep away the charges in the picture cells during time periods other than the storage period and data output period in order to enable storage of only the desired signal charges and to provide an effective output signal. Here, the analog shift register is driven to sweep away or clear the transferred charge even during the storage period. The sweeping away of unnecessary data from the shift register requires the same time period as the time period for the effective data output from the shift register. If the storage time period for the photodiode array is set to be shorter than the sweeping away time period for the shift register, and the charges stored in the photo-diode array as the effective data are transferred to the analog shift register which has not been completely cleared, the left-behind invalid data will be superimposed on or added to the effective data and the readout data from the shift register will no longer reflect the data on the text faithfully. Further reference should be made to U.S. Pat. No. 4,177,449 assigned to the present assignee.